Most of the sensory information that reaches the human brain does so through the eyes. When a person looks, he/she scans the visual environment with saccadic eye movements. He/she shifts gaze 2-3 times per second. Between two consecutive saccades the eyes hold still for a brief period, typically between 100 and 1000 ms. During this period of stable gaze (or eye fixation), visual information at the fovea is processed in more detail. The fovea is a small centre area of the human retina and has a very high visual acuity. It covers a few degrees of visual angle. So to get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception. For that reason, eye fixations (duration, location, frequency, repetition, etc.) are considered to tell how important and attended regions are for the subject.
During an eye fixation, objects in the person's periphery disappear from the person's perception (Troxler effect). To prevent this, small eye movements during fixation are produced. Several studies demonstrate that micro-saccades during fixation may be ones that prevent perception during fixation [Martinez-Conde, S., Macknick, S. L., Troncoso, X. G., & Hubel, D. H. (2009). Microsaccades: a neurophysiological analysis. Trends in Neurosciences, 32, 463-75]. However, other studies provide arguments showing that fixational micro-saccades may be laboratory artefacts and may have no role in visual functioning [Collewijn, H., & Kowler, E. (2008). The significance of microsaccades for vision and oculomotor control. Journal of Vision, 8(14), 20, 1-21]. Thus the role of micro-saccades in visual perception is still debated. Besides perception, it is debated whether micro-saccades have a role in visual attention. On the one hand, micro-saccade direction may be a reliable on-line measure of attention [Engbert, R., & Kliegl, R. (2003). Microsaccades uncover the orientation of covert attention. Vision Research, 43, 1035-45]. On the other hand, fixational eye movements may not be an index of covert attention [Horowitz, T. S., Fine, E. M., Fencsik, D. E., Yurgenson, S. and Wolfe, J. M. (2007). Fixational eye movements are not an index of covert attention. Psychological Science, 18, 356].
The eyes receive a slightly different projection of the image because of the two eyes' different positions on the head. Therefore, when looking at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. Vergence refers to the simultaneous movement of both eyes in opposite directions to obtain single binocular vision. Convergence is the simultaneous inward movement of both eyes toward each other, and divergence the simultaneous outward movement of both eyes. So to look at an object closer the eyes converge, while for an object farther away they diverge. Because of the different viewpoints observed by the left and right eye however, many other points in space do not fall on corresponding retinal locations. Visual binocular disparity defines this difference between the points of projection in the two eyes. The brain uses this binocular disparity to extract depth information from the two-dimensional retinal images. For this reason eye vergence is considered as an important visual cue for depth perception.
Methods for measuring attention and/or other more or less related cognitive behaviours are known in the prior art. Most of them try to identify and measure many different eye behaviours (e.g. saccades, blinks, eyelid, gaze fixation, pupil dilation, divergence, etc.) to obtain conclusions about attention. A drawback of this type of methods may be that they usually comprise collecting big amounts of data and performing heavy calculations on said collected data, which may cause some inefficiency and, thus, may require powerful and expensive computing resources.
Another inconvenient may be that these methods do not produce clean indicators of attention, since they take into account many different eye behaviours, which may produce measurements mixing attention with other cognitive processes different from attention (such as e.g. perception, memory, experience, etc.)
A further drawback may be that these methods may take too long to obtain more or less reliable conclusions, since they usually measure states (e.g. attention related states) considering long time scales, which may be of several minutes. These long time scales may be necessary for these methods as a consequence of that they consider different eye behaviours, some of which may introduce some disparities in the collected data. It seems to make sense to understand that collecting very big amounts of data and performing complex calculations on said data may be aimed at attenuating/compensating in some way such disparities.
For example, US2007291232A1 discloses a method of the type explained before. This method is aimed at determining mental proficiency level by monitoring point of gaze, pupillary movement, pupillary response, and other parameters in a subject performing a task, collecting the data in a database, analysing the data in the database, and assigning the subject to a score indicating the subject's particular mental proficiency level in real time. Mental proficiency may comprise the ability of paying visual attention when carrying out determined tasks. Thus, it may be understood that an object of this method is to measure attention as a parameter of the mental proficiency. This method has the previously mentioned drawbacks.